Magnetic tunnel junctions (MTJs) have a wide range of applications including tunneling magnetoresistance (TMR) hard drive read heads, magnetic sensors, magnetic random access memory (MRAM), and other spin-logic based devices. Specifically for MRAM applications, perpendicularly magnetized MTJs (p-MTJs) have attracted a considerable amount of interest due to their low writing currents using spin torque (STT-MRAM). STT-MRAM technology for writing of memory bits was described by J. Slonczewski in “Current driven excitation of magnetic multilayers”, J. Magn. Magn. Mater. V 159, L1-L7 (1996), and is known for long data retention times, and the capability to be manufactured in high density memory arrays with relatively simple architecture.
P-MTJs have a general structure wherein first and second magnetic layers contact a bottom surface and top surface, respectively, of an insulating tunnel barrier. One of the magnetic layers called the pinned layer has a magnetization fixed in an out-of-plane direction (i.e. in the +z direction). The other magnetic layer, referred to as the free layer, has a magnetization that is free to be either in the +z direction (parallel or P state) or in the −z direction (antiparallel or AP state). The difference in resistance between the P state (Rp) and AP state (Rap) is characterized by the equation (Rap−Rp)/Rp that is hereinafter referred to as DRR. It is important for p-MTJ devices to have a large DRR, as this value is directly related to the read margin for the memory bit, or how easy it is to differentiate between the P state and AP state (0 or 1 bits).
For arrays of p-MTJ devices used in embedded memory applications, controlling the distribution of both Rp and DRR values is very important. Since embedded amplification circuits only operate within a certain range of Rp and DRR values, a p-MTJ device that falls out of an acceptable range of Rp or DRR cannot be effectively read and the memory bit will fail.
A key aspect of controlling p-MTJ device performance and keeping a tight Rp and DRR distribution is the quality of the tunnel barrier. Typical tunnel barriers are comprised of one or more of MgO, Al2O3, and other metal oxides. Tunnel barriers are quite susceptible to the formation of pinholes, grain boundaries, or other “weak” points that can locally reduce the tunnel barrier height or resistance thereby resulting in p-MTJs with low resistance (RA) and low DRR. If the defects are randomly distributed, the p-MTJ array may have a few devices that are affected by the tunnel barrier defects, and have a lower Rp and lower DRR than the main population of bits, causing both reading and writing failures. These bits outside the main population are referred to as “low tail” bits.
Two methods have historically been employed for MgO formation. One is deposition of MgO using either radio-frequency (RF) sputtering or electron beam (e-beam) evaporation. A second technique is deposition of Mg followed by oxidation either through exposure to a flow of gas (natural oxidation or NOX), or through exposure to oxygen based plasmas (radical oxidation or ROX), or ozone. These two methods may lead to significantly underoxidized MgO wherein a substantial number of Mg atoms are not oxidized, which lowers DRR, or to overoxidized MgO layers where loosely bound oxygen is free to diffuse during subsequent processing and cause adjacent magnetic layers to be oxidized, which also degrades device performance.
Thus, a new MgO formation method is required where the number of defects attributed to underoxidation or overoxidation is substantially reduced. In other words, a technique is desired to enable production of a MgO layer that has a 1:1 Mg:O ratio and containing no excess oxygen to prevent oxygen diffusion into adjacent layers during subsequent processing.